In order to obtain a high purification rate with an exhaust gas purification catalyst (three-way catalyst) of an internal combustion engine, it is necessary for the air/fuel ratio of the exhaust gas to be in the vicinity (purification window) of a stoichiometric air/fuel ratio. Therefore, the air/fuel ratio of the exhaust gas is detected by an air/fuel ratio sensor that is installed on an upstream side of the exhaust gas purification catalyst, and air/fuel ratio feedback control is performed that corrects a fuel injection quantity so that the air/fuel ratio becomes the stoichiometric air/fuel ratio.
A technique is also being widely used in which a sub-exhaust gas sensor that is constituted by an oxygen sensor or the like is further provided on a downstream side of an exhaust gas purification catalyst, and sub-feedback control for supplementing the air/fuel ratio feedback control (main feedback control) is performed based on an output of the sub-exhaust gas sensor. Since the effect of a shift in the output of the air/fuel ratio sensor can be corrected by performing sub-feedback control, the air/fuel ratio of the internal combustion engine can be controlled so as to equal the stoichiometric air/fuel ratio with a high degree of accuracy.
However, at a time of engine start-up, since the air/fuel ratio sensor and the sub-exhaust gas sensor are not activated, air/fuel ratio feedback control and sub-feedback control can not be performed until the air/fuel ratio sensor and the sub-exhaust gas sensor are warmed up and activated. Because the sub-exhaust gas sensor is on the downstream side of the air/fuel ratio sensor, it takes time for the sub-exhaust gas sensor to warm up, and consequently the sub-exhaust gas sensor is activated later than the air/fuel ratio sensor. Therefore, the air/fuel ratio feedback control is performed based on only the output of the air/fuel ratio sensor during a period from activation of the air/fuel ratio sensor until activation of the sub-exhaust gas sensor.
Japanese Patent Laid-Open No. 2009-114992 discloses a technique that determines fuel properties by utilizing a fact that the characteristics of a shift (cold chute) in an output generated at an early stage after warming up of an air/fuel ratio sensor starts differ according to the fuel properties (alcohol concentration and the like). The term “cold chute” refers to a phenomenon that is thought to occur as a result of organic substances in unburned gas that remains inside an exhaust gas passage when the engine is stopped adhering to a sensor element, and then reacting at a time of engine start-up. When warming up of the air/fuel ratio sensor is completed, the cold chute phenomenon ends.